io_uring: let to set a range for file slot allocation

From recently io_uring provides an option to allocate a file index for
operation registering fixed files. However, it's utterly unusable with
mixed approaches when for a part of files the userspace knows better
where to place it, as it may race and users don't have any sane way to
pick a slot and hoping it will not be taken.

Let the userspace to register a range of fixed file slots in which the
auto-allocation happens. The use case is splittting the fixed table in
two parts, where on of them is used for auto-allocation and another for
slot-specified operations.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/66ab0394e436f38437cf7c44676e1920d09687ad.1656154403.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/io_uring_types.h

@@ -233,6 +233,9 @@ struct io_ring_ctx {
 
 	unsigned long		check_cq;
 
+	unsigned int		file_alloc_start;
+	unsigned int		file_alloc_end;
+
 	struct {
 		/*
 		 * We cache a range of free CQEs we can use, once exhausted it

include/uapi/linux/io_uring.h

@@ -449,6 +449,9 @@ enum {
 	/* sync cancelation API */
 	IORING_REGISTER_SYNC_CANCEL		= 24,
 
+	/* register a range of fixed file slots for automatic slot allocation */
+	IORING_REGISTER_FILE_ALLOC_RANGE	= 25,
+
 	/* this goes last */
 	IORING_REGISTER_LAST
 };
@@ -595,4 +598,14 @@ struct io_uring_sync_cancel_reg {
 	__u64				pad[4];
 };
 
+/*
+ * Argument for IORING_REGISTER_FILE_ALLOC_RANGE
+ * The range is specified as [off, off + len)
+ */
+struct io_uring_file_index_range {
+	__u32	off;
+	__u32	len;
+	__u64	resv;
+};
+
 #endif

io_uring/filetable.c

@@ -16,7 +16,7 @@
 static int io_file_bitmap_get(struct io_ring_ctx *ctx)
 {
 	struct io_file_table *table = &ctx->file_table;
-	unsigned long nr = ctx->nr_user_files;
+	unsigned long nr = ctx->file_alloc_end;
 	int ret;
 
 	do {
@@ -24,11 +24,10 @@ static int io_file_bitmap_get(struct io_ring_ctx *ctx)
 		if (ret != nr)
 			return ret;
 
-		if (!table->alloc_hint)
+		if (table->alloc_hint == ctx->file_alloc_start)
 			break;
-
 		nr = table->alloc_hint;
-		table->alloc_hint = 0;
+		table->alloc_hint = ctx->file_alloc_start;
 	} while (1);
 
 	return -ENFILE;
@@ -175,3 +174,20 @@ int io_fixed_fd_remove(struct io_ring_ctx *ctx, unsigned int offset)
 	io_rsrc_node_switch(ctx, ctx->file_data);
 	return 0;
 }
+
+int io_register_file_alloc_range(struct io_ring_ctx *ctx,
+				 struct io_uring_file_index_range __user *arg)
+{
+	struct io_uring_file_index_range range;
+	u32 end;
+
+	if (copy_from_user(&range, arg, sizeof(range)))
+		return -EFAULT;
+	if (check_add_overflow(range.off, range.len, &end))
+		return -EOVERFLOW;
+	if (range.resv || end > ctx->nr_user_files)
+		return -EINVAL;
+
+	io_file_table_set_alloc_range(ctx, range.off, range.len);
+	return 0;
+}

io_uring/filetable.h

@@ -3,9 +3,7 @@
 #define IOU_FILE_TABLE_H
 
 #include <linux/file.h>
-
-struct io_ring_ctx;
-struct io_kiocb;
+#include <linux/io_uring_types.h>
 
 /*
  * FFS_SCM is only available on 64-bit archs, for 32-bit we just define it as 0
@@ -33,6 +31,9 @@ int __io_fixed_fd_install(struct io_ring_ctx *ctx, struct file *file,
 				unsigned int file_slot);
 int io_fixed_fd_remove(struct io_ring_ctx *ctx, unsigned int offset);
 
+int io_register_file_alloc_range(struct io_ring_ctx *ctx,
+				 struct io_uring_file_index_range __user *arg);
+
 unsigned int io_file_get_flags(struct file *file);
 
 static inline void io_file_bitmap_clear(struct io_file_table *table, int bit)
@@ -71,4 +72,17 @@ static inline void io_fixed_file_set(struct io_fixed_file *file_slot,
 	file_slot->file_ptr = file_ptr;
 }
 
+static inline void io_reset_alloc_hint(struct io_ring_ctx *ctx)
+{
+	ctx->file_table.alloc_hint = ctx->file_alloc_start;
+}
+
+static inline void io_file_table_set_alloc_range(struct io_ring_ctx *ctx,
+						 unsigned off, unsigned len)
+{
+	ctx->file_alloc_start = off;
+	ctx->file_alloc_end = off + len;
+	io_reset_alloc_hint(ctx);
+}
+
 #endif

io_uring/io_uring.c

@@ -3866,6 +3866,12 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 			break;
 		ret = io_sync_cancel(ctx, arg);
 		break;
+	case IORING_REGISTER_FILE_ALLOC_RANGE:
+		ret = -EINVAL;
+		if (!arg || nr_args)
+			break;
+		ret = io_register_file_alloc_range(ctx, arg);
+		break;
 	default:
 		ret = -EINVAL;
 		break;

io_uring/rsrc.c

@@ -1012,6 +1012,8 @@ int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
 		io_file_bitmap_set(&ctx->file_table, i);
 	}
 
+	/* default it to the whole table */
+	io_file_table_set_alloc_range(ctx, 0, ctx->nr_user_files);
 	io_rsrc_node_switch(ctx, NULL);
 	return 0;
 fail: